The recently discovered structural transformation of the duplex form of DNA into the triplex DNA structures has been extensively analyzed during the last two years. The triplexes have been found in the following basic forms: (a) intramolecular pyrimidine-purine-pyrimidine (PyPuPy) triplexes (so-called H-forms of DNA);(b) intramolecular pyrimidine-purine-purine (PyPuPu) triplexes (*H forms);(c) intermolecular PyPuPy triplexes; and (d) intermolecular PyPuPu triplexes.Homopurine-homopyrimidine sequences are believed to be of biological importance as they are abundant in eukaryotic genomes. At the same time, these sequences are potential sites for triplex formation, and the homopyrimidine stretches in the B-form of DNA are a good target for pyrimidine dimer formation by UV-irradiation. However, because the process of dimerization in the triplexes is not well understood, analysis of the influence of triplex formation on the yield of major UV-photoproducts is relevant. The importance of this work is further strengthened by the discovery that the duplex parts of the triplex are found in the A-form. It has been recently established that the formation of the major UV-photoproduct, pyrimidine dimers, is hampered in A-form DNA. Thus, it is conceivable that triplex formation will drastically alter the sensitivity of the DNA to UV-irradiation,The recent data indicate that the triplex formation dramatically decreases the yield of pyrimidine dimers in the DNA duplex; thus a photofootprinting assay appears to be a very promising method to detect DNA triplexes. The significant advantage of this assay, as compared with other well-developed methods of triplex detection (e.g. gel electrophoresis, chemical modification and chemical and enzymatic footprinting), stems from the possibility for direct application of this method in the living cell. Hence, the long term goal of this project is to develop an assay which would make it possible to detect triplexes in living cells.